The rapid spread of the Internet has triggered the emergence of services that market commercial products via the Internet. Such a service is called an on-line sales service. In some services, when a service user inputs an authentication code such as a coupon number or the like, given in advance, a Web server provides the perk such as a discount of a given amount to the user.
Upon providing services using on-line coupons, a prevention measure against illicit use of a coupon number by a malicious user must be taken. A conventional server gives the perk upon reception of a correct number input to a remote computer. Hence, a malicious user can receive the perk by inputting a large number of different coupon numbers at random. As a result, the service provider gives the benefits to unintended users, and suffers involuntary losses.
In order to prevent such illicit use, a complicated number to which a dummy number is appended or which is encrypted is assigned as a coupon number. As a result, a malicious user is forced to input a large number of numbers until he or she can acquire a correct coupon number.
However, the increase in computer's processing power upon speeding up of computers in recent years has allowed some malicious users to make use of computer programs to easily issue and input a large number of numbers and to illicitly acquire a coupon number.
As a measure against such illicit methods, a complicated coupon number may be used. However, as the coupon number is complicated, the number of requests issued until a malicious user illicitly acquires a coupon number increases. As a result, the load on the server that provides services becomes heavier, and authentic users who normally use the services may suffer disadvantages such as deterioration of performance and the like.
Also, when the coupon number is complicated, input errors by authentic users increase.
It is not easy for the administrator who provides services to determine whether an input error of a coupon number is made due to illicit use of the number by a malicious user or a coupon number is merely erroneously input by a user who is entitled to use the coupon number.
In order to prevent illicit use, like a cash withdrawal service using an ATM (automated teller machine), it is determined that an input by a malicious user is made if the number of wrong input for a corresponding ID exceeds a predetermined number, the service is suspended. However, when the coupon number is complicated and input errors of authentic users increase, the service use of many authentic users could be suspended, generating numerous inquiries to the administrator, and thus, resulting in huge management cost.